1. Field of the Invention
This invention relates to a method for analyzing vibration property of a member, such as a head suspension, including a piezoelectric element.
2. Description of the Related Art
In a hard disk drive (also referred to as “HDD”), a head suspension is attached to a carriage and supports a head including a read/write element. In recent years, high-performance HDDs are required, and due to this, it is important to control vibration property of a head suspension.
To control the vibration property, transfer function of the head suspension is calculated with frequency response analysis. The frequency response analysis employs mode superposition method in order to reduce calculation time.
The mode superposition method assumes that the vibration caused by external force can be described by superposing two solutions; forced vibration solution largely depending on the frequency of external force, and natural vibration solution based on specific vibration to each object. By using the mode superposition method, the calculation time for the frequency response analysis can be significantly reduced.
In recent years, there are also HDDs employing a dual actuator system with a dual actuator-type head suspension. Such dual actuator system has a piezoelectric element in addition to a voice coil motor that drives a carriage. The piezoelectric element is made of, for example, lead zirconate titanate (PZT) and is fixed at the part between a base plate and a load beam, or at a tongue of the head suspension. Therefore, the head can be moved slightly in a sway direction relative to the base plate with the piezoelectric element deformed in response to voltage applied thereto.
For the dual actuator-type head suspension, the frequency response analysis based on the applied voltage to the piezoelectric element is also required.
FIGS. 10 and 11 are perspective views respectively illustrating external force application and voltage application to a head suspension 101 having a piezoelectric element. The head suspension 101 comprises a base plate 103, a load beam 105 and a flexure 109 supporting a head 107 on a tongue. The piezoelectric element is fixed at the tongue of the flexure 109.
The head suspension 101 is fixed to the carriage through the base plate 103 and is turned by the voice coil motor. Based on this structure, Y-directional 0.1 gf exciting force is applied to the head suspension 101 for the frequency response analysis as illustrated in FIG. 10. In this case, X and Z-directional exciting forces applied for the base plate 103 are kept as “0”, and also Z-directional exciting force applied for the head 107 is kept as “0”.
In the case that Y-directional 0.1 gf exiting force is applied for the head suspension, the vibration property is obtained by the frequency response analysis within the short time such as within 10 minutes, by using above-mentioned mode superposition method.
On the other hand, the head suspension 101 slightly moves the head 107 with the piezoelectric element to which voltage is applied. Based on this structure, voltage of 0.1V is applied to the piezoelectric element for the frequency response analysis. In this case, X, Y and Z-directional exciting forces applied for the base plate 103 are kept as “0”, and also Z-directional exciting force applied for the head 107 is kept as “0”.
The frequency response analysis of the piezoelectric analysis of the piezoelectric element, however, generally requires direct time integration method, also called full method, in which equation of motion is directly solved. This is because of that the piezoelectric analysis of the piezoelectric element is a coupled analysis between electric field and structure and the mode superposition method cannot be used for the frequency response analysis of the piezoelectric analysis.
The full method requires equation of motion to be solved for each frequency and significantly long time for calculation.
For example, in case that the equation of motion is solved for each 100 Hz up to 50 kHz, it requires 500 times calculation, 50,000 divided by 100. Each calculation requires 3 minutes, so that 500 times calculation requires 500 times 3 minutes, in other word 1,500 minutes or 25 hours, in total.
On the other hand, when using mode superposition method, modal analysis is previously performed to calculate mode vector and so on.
In the piezoelectric analysis of piezoelectric element, the mode superposition method involves two modes to be calculated. One of the two modes is short-circuit condition also referred to as resonant condition in which no voltage is applied for both electrode of the piezoelectric element. Another mode is open-circuit condition also referred to as antiresonant condition in which voltage is applied for one of two electrodes.
For calculating above-mentioned two modes, it requires different constraint conditions, therefore the mode superposition method cannot be applied for calculating the two modes.
The mode superposition method may be applied for calculating transfer function by using the result of one of the two modes, resonant condition mode and antiresonant condition mode. This reduces the accuracy of the calculation.